Yase governs the Mirror Capital alongside her two siblings, originally created as a drawing by Myōe alongside her brother Kurama. Explicitly an oni, she resides in an estate within the forbidden zone outside the city. Her typical demeanor is polite, refined, and wealthy, frequently hosting elaborate tea parties, yet she possesses a volatile temper; when angered, she transforms dramatically into a raging, destructive monster.

Her deep adoration for her mother, Lady Koto, profoundly shapes her character. Following her parents' unexplained disappearance, Yase copes with abandonment trauma by obsessively fixating on the past, hoarding physical objects and memories associated with her family, particularly Lady Koto. This reflects her struggle to preserve a lost era, rendering her emotionally unstable and prone to repression. Her visual design incorporates stylistic elements from 1970s–1980s shōjo manga, resembling Yumiko Igarashi's work with large, expressive eyes, further emphasizing her connection to the past.

Within the family dynamic, Yase serves as a counterpoint to her siblings: Kurama fixates on escaping the Mirror Capital (the future) while Yakushimaru remains mired in detached resentment (the present); Yase embodies an inability to move beyond the past. Her character arc involves subtle maturation sparked by interactions with Koto, the younger girl who arrives in the Mirror Capital. A physical confrontation with Koto steers Yase towards greater emotional stability, allowing her to begin reflecting on surrounding kindness and developing a faint, more sympathetic aura over time, signifying gradual growth.

Thematically, Yase illustrates the Buddhist concept of *dukkha* (suffering from impermanence) and the struggle to accept change. Her hoarding and mood swings symbolize the pain of clinging to transient things and idealized memories, positioning her as a representation of the *puer aeternus* (eternal child) who avoids confronting reality.